1. Field of the Invention
The present invention relates to a short dwell coater apparatus. More particularly, the present invention relates to a short dwell coater apparatus for coating a web with a coating material, the web moving contiguously with a rotating backing roll.
2.Information Disclosure Statement
Before the introduction of short dwell coaters, the coating of a paper web, or the like, was accomplished by rotating a roll within a pan of coating material. The coating material was offset from the roll rotating within the pan to an offset roll. Thereafter, the coating material was transferred to a web moving past and in contact with the coating material on the offset roll. More particularly, the coating material was applied to the web by passing the web through a coating nip defined between the offset roll and a backing roll.
Although pressures in excess of ten pounds per square inch were typically applied at the coating nip, thereby providing sufficient pressure between the coating material and the web, the coating applied to the web suffered from the problem of nonuniformity in a cross-machine direction. Such nonuniformity was mainly due to the uneven distribution of coating material in a cross-machine direction on the offset roll.
Additionally, because of the relatively long residence time of the moving web relative to the coating material, excess quantities of coating material were applied to the web.
With the advent of the so-called "short dwell coater", the aforementioned problems were overcome. The short dwell coater essentially includes a housing defining a coating chamber supplied with a pressurized source of coating material. The web moves between the coating chamber and a backing roll such that coating material is applied to the moving web. A resilient blade is disposed immediately downstream relative to the coating chamber, the blade defining the downstream end of the chamber.
In the prior art, relatively low pressures were applied within the coating chamber, such pressures being less than two pounds per square inch, so that as the web moved past the coating chamber, a very thin coating was applied to the moving web. Due to the aforementioned low coating pressure and the short residence time of the web moving past the coating chamber, the coating material applied to the web was relatively thin with little penetration of the coating material into the fibers of the resultant coated web. Furthermore, the resilient blade metered and smoothed the coating material onto the moving web.
However, with the prior art, short dwell or "short residence time" coaters there exists a problem known as mottling, or streaking, of the resultant coated web. It is understood that such mottling or streaking is caused by the relatively low application pressure within the coating chamber.
According to the present invention, it was discovered that with the aforementioned roll-coating procedure, a relatively high application pressure above ten pounds per square inch permitted absorption of water within the coating material to penetrate the fibers of the moving web, leaving a residual layer of concentrated coating particulate material at the surface. Whereas, in the case of the aforementioned short dwell coater apparatus utilizing a pressure below two pounds per square inch, insufficient application pressure was available to effect adequate bonding between the coating material and the moving web.
In the aforementioned short dwell coaters, if the pressure within the coating chamber were increased in order to reduce mottling and streaking, the increased pressure of the coating material within the coating chamber would have unduly affected the pressure exerted on the resilient blade because the resilient blade was located within the coating chamber.
The present invention seeks to overcome the aforementioned problem by providing a coating chamber with an upstream and a downstream overflow gap so that a relatively high pressure can be applied within the coating chamber, in order to overcome the problem of mottling and streaking. The resilient blade is disposed downstream relative to the downstream overflow gap so that the relatively high pressure of the coating material within the coating chamber does not affect the pressure of the coating material contacting the resilient blade. According to the present invention, the pressure within the coating chamber is within the range of ten to fifty pounds per square inch, whereas the pressure of the coating material between the second or downstream overflow gap and resilient blade is substantially atmospheric. By this means, mottling and streaking of the resultant coated web is inhibited while minimum pressure is applied to the resilient blade, resulting in greater controllability of the resilient blade.
Therefore, it is a primary object of the present invention to overcome the inadequacies of the prior art proposals and to provide a short dwell coater apparatus that provides a considerable contribution to the art of coating a moving web of paper.
Another object of the present invention is the provision of a short dwell coater apparatus which includes a housing defining a coating chamber having an upstream and a downstream end. Said upstream and downstream ends of the coating chamber define first and second weirs extending in a cross-machine direction such that the coating chamber is bounded by the weirs and the web moving between the weirs.
Another object of the present invention is the provision of a short dwell coater in which the first and second weirs define respectively first and second overflow gaps disposed between the respective weirs and the web moving past the respective weir. The arrangement is such that during use of the apparatus when the web moves past a coating chamber, coating material is coated on the web while excess coating material overflows through the first and second gaps for recirculation through the coating chamber.
Another object of the present invention is the provision of a short dwell coater apparatus in which the resilient coating blade is disposed downstream relative to the second weir such that the pressure exerted by the coating material on the blade is less than the pressure of the coating material within the coating chamber, so that mottling and streaking of the resultant web is inhibited while maintaining minimum pressure on the blade.
Another object of the present invention is the provision of a short dwell coater apparatus which includes overflow control means connected to the housing for controllably urging the housing and the first and second weirs radially relative to the backing roll, for controlling the first and second gaps such that the pressure within the coating chamber is controlled. The arrangement is such that when the weirs are urged towards the web, the gaps are decreased and the pressure within the coating chamber is increased.
Another object of the present invention is the provision of a short dwell coater apparatus which includes overflow control means including a frame, the frame defining a cylinder controllably connected to a source of hydraulic pressure, and a piston slidably cooperating with the cylinder. A housing also includes a pivotal bearing which pivotally cooperates with the piston such that the housing is controllably moved relative to a backing roll for controlling the pressure within the coating chamber.
Another object of the present invention is the provision of a short dwell coater apparatus which includes a frame and an overflow control means disposed between the frame and the housing for controllably moving the housing relative to the frame for controlling the pressure within the coating chamber. A blade includes a proximal and a distal end, with the proximal end being secured to the frame and the distal end cooperating with the applied coating material. Additionally, the apparatus includes regulating means disposed between the proximal and distal ends of the blade. The regulating means cooperates with the blade and the frame for regulating the pressure applied by the distal end of the blade on the applied coating. Pressure applied to the distal end of the blade by the applied coating is substantially the same pressure as the pressure of the coating material being recirculated from the second gap to the coating chamber.
Another object of the present invention is the provision of a short dwell coater apparatus in which the pressure within the coating chamber is within the range ten to fifty pounds per square inch, and the pressure of the coating material overflowing through the second gap and contacting the blade is substantially atmospheric, such that the pressure of the coating material contacting the blade is unaffected by the pressure within the coating chamber.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by consideration of the detailed description contained hereinafter, taken in conjunction with the annexed drawings.